The present invention relates to rotation sensors for use in, e.g., gyroscopes, and particularly to fiber optic rotation sensors.
Fiber optic rotation sensors typically comprise a loop of single-mode optical fiber to which a pair of lightwaves are coupled for propagation in opposite directions around a loop. If the loop is rotated, the counter-propagating waves will undergo a phase shift, due to the well-known Sagnac effect, yielding a phase difference between the waves after traverse of the loop. By detecting this phase difference, a direct indication of the rotation rate of the loop may be obtained.
If the optical path lengths about the loop for the counter-propagating waves are equal when the loop is at rest, the interferometer is said to be "reciprocal". In practice, however, fiber interferometer loops are ordinarily not reciprocal, due to the fact that present, commercially available optical fibers are not optically perfect, but are birefringent (i.e., doubly refractive), resulting in two orthogonal polarization modes, each of which propagates light at a different velocity. One of the polarization modes, therefore, provides a "fast channel", while the other provides a "slow channel." In addition, the fiber birefringence is sensitive to environmental factors, such as temperature, pressure, magnetic fields, etc., so that, at any given point along the fiber, the birefringence can vary over time in an unpredictable manner. Birefringence affects the counter-propagating waves in a complex way, however, the effect may be viewed as causing a portion of the waves to be coupled from one of the polarization modes to the other, i.e., from the "fast channel" to the "slow channel or vice versa. The result of such coupling between modes is that each of the counter-propagating waves may travel different optical paths around the loop, and thus, require different time periods to traverse the fiber loop, so that there is a phase difference between the waves when the loop is at rest, thereby making the interferometer non-reciprocal.
The foregoing may be more fully understood through a rather simplistic, extreme example in which it is assumed that there is birefringence only at one point in the fiber loop, and that this point is located near one end of the loop. It is also assumed that such birefringence is sufficient to cause light energy to be entirely coupled from one polarization mode to the other, and that there is no coupling between modes anywhere else in the fiber loop. If the counter-propagating waves are introduced into the loop in the fast channel, one of the waves will immediately be coupled to the slow channel while the other wave will traverse most of the loop before being coupled to the slow channel. Thus, one of the waves will traverse most of the loop in the fast channel, while the other will traverse most of the loop in the slow channel, yielding a phase difference between the waves when the loop is at rest. If this birefringence-induced phase difference were constant, there would, of course, be no problem, since the rotational induced Sagnac phase difference could be measured as a deviation from this constant birefringence-induced phase difference. Unfortunately, however, such birefringence-induced phase differences vary with time, in an unpredictable manner, and thus, these birefringence-induced phase differences are indistinguishable from rotationally-induced, Sagnac phase differences. Thus, time varying changes in birefringence are a major source of error in fiber optic rotation sensors.
The prior art has addressed the problem of nonreciprocal, birefringence-induced phase differences in a variety of ways. In one approach, described by R. A. Bergh, H. C. Lefevre, and H. J. Shaw in Optics Letters, Volume 6, No. 10 (October 1981), a fiber optical polarizer is utilized to block light in one of the two orthogonal polarization modes while passing light in the other. This insures that only a single optical path is utilized, thereby providing reciprocity. This approach is also described in International Patent Application No. PCT/US 82/00400 published Oct. 14, 1982, as Publication No. WO 82/03456, entitled "Fiber Optic Rotation Sensor," and also in corresponding U.S. patent application Ser. No. 307,095, filed Sept. 30, 1981, entitled "Fiber Optic Rotation Sensor", which is a continuation-in-part of patent application Ser. No. 249,714, filed Mar. 31, 1981. Another approach involves utilizing unpolarized light, which has been found to result in cancellation of birefringence-induced phase differences upon combining the counter-propagating waves after traverse of the loop. The degree of cancellation is proportional to the degree to which the light waves are unpolarized. This approach is described in detail in International Patent Application No. PCT/US 82/00985, published Feb. 17, 1983 as Publication No. 83/00552, and also in corresponding U.S. patent application, Ser. No. 288,212, filed Jul. 29, 1981, entitled "Fiber Optic Rotation Sensor Utilizing Unpolarized Light".
It is also known in the art to utilize polarization-conserving fibers to reduce coupling between the modes. Polarization-conserving fibers are essentially high birefringence fibers, in which the fiber is mechanically stressed during manufacture to increase the difference in the refractive indicies of the two polarization modes. This reduces coupling between the modes, since the high birefringence tends to preserve the polarization of the light waves. In effect, changes in birefringence due to environmental factors are overwhelmed by the stress-induced birefringence created during manufacture of the fiber.